The most common way of assembling a tactile crystal, generally made of sapphire, on a wristwatch case is explained hereinafter with reference to FIG. 1 and to the schematic cross-section of FIG. 2, in which the contrasts and thicknesses have been greatly exaggerated for better comprehension.
The wristwatch shown includes, in a known manner, a case 1 formed of a middle part 3 closed on its bottom part by a back cover 5 and on its top part by a crystal 10 delimiting a compartment 7. Compartment 7 is for housing a watch movement 8 for the display of timer data on a dial 6. In the example shown, the display is an analogue display by means of hands for the hours 6a, minutes 6b and seconds 6c. 
It can also be seen that the bottom face 11 of crystal 10 includes five electrodes 15 forming, for example, capacitive sensors, the electrodes 15 each being connected via conductive paths 16 to contact zones 17 made close to the edge 12 of crystal 10. The conductive network 15, 16, 17 is made in a known manner by structuring a transparent conductive oxide, such as indium and tin oxide (ITO), deposited for example by vapour deposition and having a thickness comprised between 25 and 75 nm. Electrodes 15 and conductive paths 16 can be made practically invisible by depositing dielectric layers in the spaces comprised between the electrodes 15 and paths 16, as disclosed for example in EP Patent No. 1 457 865. However, contact zones 17, which are placed in contact with a connector 19 passing through an insulating connector ring 2 and connecting an electronic module 9, capable of detecting the presence of a finger above one electrode 15 and of sending a control signal to movement 8, are much more visible.
When crystal 10 is assembled on case 1 by means of a bezel 13 secured to the middle part, bezel 13 need only be given sufficient width to conceal contact zones 17.
However, there exist cases, particularly for wristwatches, wherein crystal 10 extends to the outer edge of middle part 3. Crystal 10 is assembled for example via bonding on the shoulder of the middle part 3 or on an insulating connector ring 2. In order to conceal the adhesive joint, metallising has to be carried out by depositing at least one very thin anchorage layer, generally by chromium evaporation, and possibly other metals or alloys, close to the edge of the crystal, in order to try to harmonise the hue of the middle part shoulder seen through the crystal with that of the rest of the middle part. For tactile crystals, there is thus a significant risk of short-circuiting two neighbouring contact zones 17, or of creating stray capacitance.